An asynchronous, clocked communication system with equidistant characteristics is taken to mean a system a system with the least two subscribers who are connected via a data network for the purposes of mutual exchange of data or mutual transmission of data. In this case data is exchanged cyclically in equidistant communication cycles which are specified by the communication clock used by the system. Subscribers are for example central automation devices, programming, project planning or operating devices, peripheral devices such as input/output modules, drives, actors, sensors, Programmable Logic Controllers (PLC) or other control units, computers or machines which exchange electronic data with other machines and process data, especially from other machines. Subscribers are also called network nodes or nodes. Control units in this document are taken to mean closed-loop controllers or control units of all types, but also switches and/or switch controllers for example. Typical examples of data networks used are bus systems such as Field Bus, Profibus, Ethernet, Industrial Ethernet, FireWire or also PC-internal bus systems (PCI), etc., but especially also the isochronous Realtime Ethernet.
Data networks allow communication between a number of subscribers by networking, that is connecting the individual subscribers to each other. Communication here means the transmission of data between the subscribers . The data to be transmitted is sent in this case as data telegrams, i.e. the data is packed into a number of packets and sent in this form over the data network to the corresponding recipient. The term data packet is thus used. The term transmission of data is used in this document fully synonymously with the above-mentioned transmission of data telegrams or data packets.
In distributed automation systems, for example in the area of drive technology, specific data must arrive at specific times at the intended subscribers and must be processed by the recipients. This is referred to as realtime-critical data or data traffic since if the data does not arrive at its intended destination at the right time this can produce undesired results at the subscriber by contrast with non-realtime critical, for example Internet or Intranet based data communication.
Realtime Ethernet communication is a planned, cyclic communication. Thus a list for the realtime telegrams to be sent (IRT telegrams) as well as a list for the IRT telegrams to be received is assigned to each realtime port. The list elements each correspond to an IRT telegram. Each list element additionally contains the information about the time at which the corresponding assigned IRT telegram must be received or is to be transmitted within the isochronous cycle (communication cycle). In an isochronous cycle the send and receive lists of all realtime-capable Ethernet ports are processed and IRT communication is thus executed in the realtime-capable Ethernet network. IRT communication must however be planned in the initialization phase and cannot change during IRT operation. This leads to an inflexible IRT communication which does not allow dynamic behavior.
Operation of subnetworks in particular without knowing the subsequent final project planning of the overall network proves to be difficult in practice. Previously, each time they have been expanded, such systems have had to be switched off, reinitialized and started up again. If the subsequent overall system is already known, smooth expansion has only been possible thus far by planning in the later IRT communication of the overall network and executing it in cyclical operation when implementing the subnetwork. A retroactive change not known about beforehand however again leads to the problems already described and to their effects. The conversion of the IRT communication of the overall network because of changed peripheral conditions of the system or the production has also been undertaken previously by switching off, reinitialization and starting up the entire IRT communication of the system again.
A equidistant, deterministic, cyclic exchange of data in communication systems is based on a common clock or time basis of all components involved in communication. The clock or time basis is transmitted by a special component (clock pulse generator) to the other components. With the isochronous realtime Ethernet the clock or time basis is prespecified by a synchronization master which sends synchronization telegrams.
Communication with realtime-capable Ethernet components is divided up into cyclical, planned IRT communication and non-cyclical and non-planned NRT (Non Realtime) communication. NRT communication corresponds in this case to the current normal and general Ethernet traffic known as Ethernet communication. The cyclical realtime communication as well as the non-cyclical non-realtime communication is executed in each isochronous cycle (communication cycle). The IRT send and IRT receive lists for realtime communication are stored in a communication memory.
A system and a method for transmission of data over switchable data networks, especially the Ethernet, is published in German Patent Application DE 100 58 524.8, which allows mixed operation of realtime-critical and non-realtime-critical, especially Internet or Intranet-based, data communication.